(1) Field of the Invention
This invention relates to a tire-reinforcing dip cord. More particularly, it relates to a tire-reinforcing dip cord which has a high strength, a high toughness and a high softness and which can reduce the weight of a tire without degradation of the properties of the tire.
(2) Description of the Prior Art
A tire-reinforcing material is ordinarily used in the form of a cord fabric. A dip cord constitutes a warp of this cord fabric, and the dip cord is ordinarily prepared by imparting first and second twists to a plurality of yarns, applying an adhesive, for example, an aqueous dispersion formed by adding a rubber latex to a polyhydric phenol/formaldehyde condensate (hereinafter referred to as "RF/L" for brevity) to the resulting cord and heat-setting RF/L.
Among tire-reinforcing materials, nylon fibers have excellent strength, durability and heat resistance. Accordingly, tire-reinforcing materials composed of nylon fibers are used for large tires for trucks or buses, construction vehicles, air crafts and the like. However, in case of these large tires, the amount of the reinforcing material used for each tire is much larger than in case of tires for ordinary passenger cars, and a large quantity of energy and a large fuel expense are necessary.
With recent increase in the price of the fuel, it is desired to save the energy quantity and the fuel expense, and it also is eagerly desired to reduce the weight of a tire by reducing the number of plies or ends of the reinforcing material. If this reduction of the tire weight is attained, it is expected to improve the fatigue resistance because of reduction of the quantity of heat generated during driving and also improve the productivity at the tire-forming step. Accordingly, it is eagerly desired to develop nylon fibers having a much higher strength than the nylon fibers used in this field at the present.
It is known that a nylon fibrous material having high strengths can ordinarily be prepared by spinning a polymer having a high degree of polymerization and drawing the spun fibers at a high draw ratio. When fibers drawn at a high draw ratio are used as a tire-reinforcing material, the strengths are reduced at the dipping step or at the vulcanizing step of integrating the fibers with a rubber. Furthermore, the obtained dip cord is extremely hard, and the operation adaptability is reduced at the tire-forming step and the toughness having influences on the properties of the tire such as the durability and impact resistance is drastically reduced. Incidentally, the toughness means the area of the load-elongation curve but for convenience's sake, the toughness is calculated according to the equation of (strength at break).times.(elongation at break).times.1/2.
We made various attempts to increase the strength of nylon fibers per se. However, if the customary dipping method is adopted, the strength is not increased to a level allowing reduction of the weight of a tire even when such high strength nylon fibers are used, and the tire-reinforcing dip cord is rigid and has a poor toughness.
Accordingly, we furthered our researches and found that if certain specific properties are given to a dip cord by adopting specific means, the weight of a tire can be reduced without deterioration of the properties of the tire. Namely, at first, we noted a phenomenon in which the strength at break of a dip cord is improved by mechanical softening means, for example, by rubbing the cord under tension on an edge of an article. In the conventional cord-dipping method, an adhesive permeates into the interior of the cord to form a adhesive layer between filaments in the peripheral portion of the cord. The outermost adhesive layer is effectively utilized for bonding the cord to a rubber. However, the adhesive among the filaments strongly restricts the movements of the filaments, resulting in increase of the hardness. In the dip cord obtained according to this conventional method, only the bonding property is taken into consideration, and the dip cord contains an excess of the adhesive making no contribution to the bonding property. This excess of the adhesive inhibits the movement of the filaments corresponding to the tension at the measurement of the strength of the dip cord and increases the non-uniform strain in the section of the dip cord, with the result that the strength and elongation characteristics are reduced.